The present invention relates to a method for manufacturing an electrolytic capacitor, and an electrolytic capacitor manufactured with this method, the use of this electrolytic capacitor, and electronic circuits.
A standard electrolytic capacitor generally consists of a porous metal electrode, an oxide layer disposed on the metallic surface, an electrically conductive material, generally a solid, which is introduced into the porous structure, an external electrode (contact), such as a silver layer, and other electrical contacts and an encapsulation. One frequently used electrolytic capacitor is the tantalum electrolytic capacitor, of which the anode electrode is made from the valve metal tantalum, on which a uniform, dielectric layer of tantalum pentoxide has been produced through anodic oxidation (also referred to as “forming”). A liquid or solid electrolyte forms the cathode of the capacitor. Aluminium capacitors, of which the anode electrode is made from the valve metal aluminium, on which a uniform, electrically insulating aluminium oxide layer is produced as a dielectric by means of anodic oxidation, are also frequently used. Here also, a liquid electrolyte or a solid electrolyte form the cathode of the capacitor. The aluminium capacitors are generally embodied as wound capacitors or stack-type capacitors.
In view of their high electrical conductivity, π-conjugated polymers are particularly suitable as solid electrolytes in the capacitors described above. π-conjugated polymers are also referred to as conductive polymers or as synthetic metals. They are gaining increasing commercial significance because, by comparison with metals, polymers have advantages with regard to processing, weight and the selective adjustment of properties through chemical modification. Examples of known π-conjugated polymers include polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes), wherein poly(3,4-ethylenedioxythiophene) (PEDOT) is a particularly important polythiophene which is used technically, because it has a very high conductivity in its oxidised form.
The application of the solid electrolytes based on conductive polymers to the oxide layer can be performed in different ways. For example, EP-A-0 340 512 describes the manufacture of a solid electrolyte from 3,4-ethylenedioxythiophene and its use in electrolyte capacitors. According to the teaching of this specification, 3,4-ethylenedioxythiophene is polymerised in situ on the oxide layer. After the depositing of the polymer solid electrolyte, the oxide layer of the capacitor must generally be reformed in order to achieve low residual currents, as described, for example, in EP-A-0 899 757. For this purpose, the capacitor is immersed in an electrolyte and exposed to an electric voltage, which does not exceed the anodisation voltage of the oxide film.
However, the disadvantage of manufacturing solid electrolytic capacitors using in situ polymerisation is to be seen, inter alia, in the complexity of the process. Accordingly, a polymerisation process, which includes respectively the method steps of immersion, polymerisation and washing, generally takes several hours. In this context, explosive or toxic solvents may under some circumstances have to be used. A further disadvantage of the in situ method for manufacturing solid electrolytic capacitors is that anions of the oxidation medium or optionally other monomer anions are used as counterions for the conductive polymer. However, because of their small size, these are not bound in a sufficiently stable manner to the polymer. Accordingly, especially at high operating temperatures of the capacitor, a diffusion of the counterions can occur and therefore an increase in the equivalent series resistance (ESR) of the capacitor. The alternative use of high-molecular polymer counterions in chemical in situ polymerisation does not achieve sufficiently conductive films and therefore does not achieve low ESR values.
In the prior art, alternative methods for manufacturing solid electrolytes based on conductive polymers in electrolytic capacitors have therefore been developed. For example, DE-A-10 2005 043828 describes a method for manufacturing solid electrolytes in capacitors, in which a dispersion comprising the already polymerised thiophene, for example, the PEDOT/PSS dispersions known from the prior art, is applied to the oxide layer, and then the dispersion medium is removed by evaporation. However, there is a requirement to increase further the breakdown voltage, which is a measure for the reliability of an electrolytic capacitor, especially for high operating voltages. The breakdown voltage is the voltage at which the dielectric (oxide layer) of the capacitor no longer withstands the electrical field strength and electrical breakdowns occur between the anode and the cathode, which leads to a short circuit in the capacitor. The higher the breakdown voltage is, the better the quality of the dielectric and therefore also the more reliable the capacitor is. Moreover, the rated voltage at which the capacitor can be used is higher, the higher the breakdown voltage of the capacitor is.
An increase of the breakdown voltage can be achieved in aluminium capacitors according to the doctrine of WO-A-2007/097364, JP 2008-109065, JP 2008-109068 or JP 2008-109069, for example, by adding ion-conducting substances, such as polyethylene glycols to the polymer dispersions which are used to manufacture the solid electrolyte layer before the application of the dispersion to the oxide layer. The disadvantage of this approach, however, is that, although the breakdown voltage of the capacitor can be improved, this improvement is associated with an undesirably strong decline in the capacitance of the capacitor at low temperatures.
In addition to the addition of polyethylene glycols to the polymer dispersion, in the context of so-called “hybrid capacitors”, which provide a combination of a solid electrolyte and a liquid electrolyte, it is also known, for example, from U.S. Pat. No. 7,497,879 B2 or JP 2009-111174, to impregnate the solid electrolytes, after they have been applied to the oxide layer, with a solution comprising γ-butyrolactone or sulfolane, in order to increase the capacitance yield and accordingly reduce the residual current in this manner. Such capacitors also show an undesirably large reduction in the capacitance at low temperatures. Furthermore, the components used in U.S. Pat. No. 7,497,879 B2 or JP 2009-111174 volatilise on increasing the temperature of the capacitor, which can occur with normal use in a component or during the manufacturing process of the capacitor, which leads to a drying-out of such a hybrid capacitor.